The subject application relates generally to lock detectors for digital communication systems. More particularly, this invention is directed to a lock detector having a trellis structure and a method for lock detection that provide indication of tracking a carrier signal. The trellis based lock detector and trellis-based method for lock detection of the present application are capable of being advantageously used in communication systems designed to convey signals employing any type of digital modulation, that are capable of being described by a trellis, such as, for example, and without limitation, pulse width modulation/frequency modulation (PCM/FM), Feher-patented quadrature-phase-shift keying (FQPSK), shaped offset quadrature-phase-shift keying (SOQPSK), and all forms of continuous phase modulation (CPM).
Previously known carrier lock detectors are typically implemented as phase lock loop detectors, delay-locked loop detectors, or signal power detectors. The performance of lock detectors of these types is dependant on the carrier signal amplitude. This being the case, at low levels of signal to noise ratio, previously known lock detectors may indicate an out-of-lock condition even though the carrier is still locked.
Also known in the art, are more advanced lock detectors that are signal amplitude independent. However lock detectors of this type exhibit high complexity.
Thus, there is a need for a carrier lock detector and method for carrier lock detection that overcome the above mentioned problems.
It would be desirable to have a carrier lock detector and method that are not dependant on the amplitude of the carrier signal.
Further, it is desirable to be able to reliably discriminate between a digitally modulated informative signal with a very poor signal to noise ratio, and no signal whatsoever.
It would be also desirable to have a carrier lock detector and method capable of providing reliable lock information even at very low signal to noise ratio, while simplifying hardware requirements.